In manufacturing a semiconductor device, a plasma etching process is repeatedly performed on a substrate such as a semiconductor wafer, a FPD substrate, or a substrate for a solar cell so as to form a micro circuit pattern thereon. In the plasma etching process, for example, plasma is generated by applying a high frequency voltage to electrodes placed to face each other within a depressurizable processing chamber of a plasma processing apparatus, and an etching process is performed on the substrate mounted on a mounting table by using the plasma.
In order to perform a uniform and favorable etching process at a peripheral portion of the substrate as well as at a central portion thereof in the plasma etching process, a focus ring is installed on the mounting table so as to surround the substrate on the mounting table (see Patent Document 1).
Generally, in such a plasma processing apparatus, there has been a problem in that the peripheral portion of the substrate is protruded from the mounting table and there is a gap between the mounting table and the focus ring. Accordingly, plasma products such as CF-based polymers may enter into the gap and be deposited on a bottom surface (rear surface) of the peripheral portion of the substrate.
In order to reduce the deposits on the rear surface of the peripheral portion of the substrate, an electric field is generated by a potential difference between the substrate (mounting table) and the focus ring during a plasma process by insulating the mounting table and the focus ring from each other via a dielectric member provided therebetween, so that ions in the plasma are attracted to and introduced into the peripheral portion of the substrate by such an electric field (see Patent Document 2).    Patent Document 1: Japanese Patent Laid-open Publication No. 2007-258417    Patent Document 2: Japanese Patent Laid-open Publication No. 2007-250967
However, as disclosed in Patent Document 2, for example, when the mounting table and the focus ring are insulated from each other via the dielectric member provided therebetween, a potential difference between the substrate (mounting table) and the focus ring is generated during a plasma process. Therefore, a thickness of a plasma sheath on the substrate is different from that of a plasma sheath on the focus ring. Accordingly, the plasma sheath is deformed, and, thus, an incident angle of ions in the plasma to be attracted to, e.g., the peripheral portion of the substrate may be inclined, resulting in inclination of holes or trenches formed in the peripheral portion.
In this case, a height of the plasma sheath on the focus ring can be adjusted by adjusting a height of a top surface of the focus ring. Therefore, it is possible to suppress deformation of the plasma sheath. For this reason, conventionally, a plurality of focus rings of which top surfaces have various heights are prepared and an optimum focus ring is selected from among the prepared focus rings by testing a plasma process thereon.
However, even if the optimum focus ring is found, the focus ring may be eroded through repeated plasma etching processes and, thus, its height may be changed and the plasma sheath may be deformed again. In such a case, the height of the focus ring cannot be minutely adjusted, and, thus, whenever a process result such as inclination of the holes or the trenches exceeds tolerance, the focus ring should be replaced. Further, if conditions of the plasma etching process are changed, a potential applied to the substrate or a state of the plasma sheath may be changed. Therefore, a focus ring having an optimum height for each process condition should be designed again.
If the height of the top surface of the focus ring is increased, inclination of the plasma sheath above the vicinity of the peripheral portion of the substrate becomes improved. Therefore, the incident angle of the ions to the vicinity of the peripheral portion of the substrate can be nearly vertical, and, thus, the inclination of the holes or the trenches formed in the peripheral portion of the substrate can be improved. However, since the incident angle of the ions in the plasma entered into the gap between the substrate and the focus ring becomes also nearly vertical, it becomes difficult for the ions to reach, e.g., a rear surface of the peripheral portion of the substrate. Accordingly, an effect of reducing the deposits on the peripheral portion of the substrate becomes decreased. As discussed above, there is a tradeoff relation between the inclination of the holes or trenches formed in the peripheral portion of the substrate and the effect of reducing the deposits on a rear surface of the peripheral portion of the substrate. Therefore, it is difficult to improve these both effects in balance only by adjusting the height of the top surface of the focus ring.
Further, it is described in Patent Document 1 that a predetermined DC voltage is applied to a focus ring based on a degree of erosion of the focus ring so as to increase a thickness of a plasma sheath on the focus ring, thereby suppressing deformation of the plasma sheath. However, in order to find an optimum voltage, it is required to detect plasma and predict or detect the degree of erosion of the focus ring. Therefore, process or configuration becomes complicated to find such an optimum voltage or its adjustment voltage. Further, since the focus ring described in Patent Document 1 is directly installed on a mounting table, it is electrically connected with the mounting table without being floated. Accordingly, it is impossible to improve the effect of reducing the deposits on the peripheral portion of the substrate.
Furthermore, it is described in Patent Document 2 that a DC voltage is applied from a variable DC power supply to a focus ring insulated from a mounting table, thereby changing a potential difference between a substrate (mounting table) and the focus ring. However, there is no description of a method of finding an optimum application voltage. Therefore, in case of finding the optimum voltage according to a conventional way as described in Patent Document 1, a process or configuration becomes complicated, and, thus, much time and effort are required. Moreover, improvement in a process result (e.g., improvement in inclination of holes or trenches) at a peripheral portion of a substrate has not been considered in Patent Document 2.